The present invention relates generally to power systems for mining haul trucks, and more particularly to a system and method for reinjection of retard energy in a trolley-based electric mining haul truck.
Mining haul trucks are typically equipped with electrical drive motors. Under some travel conditions, such as inside a mining pit, around a crusher, and on level surfaces, electrical power is supplied by an electrical generator powered by a diesel engine. Under more demanding conditions, electrical power is supplied by a trolley line. The haul truck draws electrical power from the trolley line via a pantograph. In particular, trolley lines are commonly used when a haul truck is filled with payload inside a mining pit and transports the payload along an uphill grade to the surface. During downhill travel, power supplied by the diesel engine is adequate, and there is typically no need to install a trolley line (which may be expensive) along the downhill path.
The electrical power drawn from the trolley line exhibits large dynamic swings. When the haul truck with a heavy load is accelerating on an uphill grade, for example, the peak power demand may exceed twice the average power demand. High peak power demand has a negative impact on both the electrical utility company and the mining operator. The high peak power demand may overload the electrical utility substation supplying electrical power to the trolley line. Voltage sags, or even outages, may result. High peak power demand may also overheat the trolley line cables and the pantograph contacts, leading to increased failure rates.
In addition to improved performance and reliability, there is also an economic incentive for reducing peak power demand. Electrical power utility companies supplying power to the mines typically measure the power demand of a mine based on 15-minute intervals, and billing is adjusted for peak power demand during each 15-minute interval. What are needed are method and apparatus for reducing power drawn by haul trucks from an electrical power utility. Method and apparatus which reduce wasted energy are particularly advantageous.